The controlling case is a U.S. Supreme Court case, Roper vs. Simmons (2005) in which the court held that applying the death penalty to juveniles was cruel and unusual punishment, and thereby a violation of the Fourth Amendment to the Constitution.
The decision was written by Justice Anthony Kennedy, who wrote:
When a juvenile offender commits a heinous crime, the State can exact forfeiture of some of the most basic liberties, but the State cannot extinguish his life and his potential to attain a mature understanding of his own humanity.
The Court had previously held in Stanford vs. Kentucky (1989) that execution of sixteen and seventeen year old offenders was not constitutionally barred. the Roper case overruled Stanford on the basis of "evolving standards of decency."
Justice Kennedy's opinion noted that thirty states had passed statutes prohibiting the execution of juveniles. Twelve of those states had outlawed the death penalty altogether. He also noted that juveniles are uniquely subject to the influence of others and quite often were incapable of exercising mature judgment. Thus, neither deterrence nor retribution could justify the imposition of such an extreme penalty. Justice Kennedy noted:
Retribution is not proportional if the law’s most severe penalty is imposed on one whose culpability or blameworthiness is diminished, to a substantial degree, by reason of youth and immaturity.